Newbie here!

Hi guys, I just joined this group last night. I'm 24 and we are ttc#1- this is our first month! AF is due 9/23 so I'm in my tww. Question though- and it's probably a dumb one, but how exactly do you guys figure how many dpo you are? Do you go by from when glow said you should ovulate, or the end of your fertile week? I'm driving myself crazy guessing!
I got a pos opk last Tuesday.